1. Field of the Invention
The present invention relates to an illumination optical arrangement employing a laser light source to produce high brightness illumination which is substantially free of any interference patterns, and more particularly the invention relates to an illumination optical arrangement suitable for example for an exposure apparatus used in the fabrication of semiconductor integrated circuits such as VSLIs.
2. Description of the Prior Art
In an apparatus for effecting the photolithography used for the manufacture of semiconductor integrated circuits, e.g., VSLIs, it is necessary to effect uniform illumination of high brightness, and generally a super-high pressure mercury lamp is frequently used for such illumination.
However, due to the recent trend toward higher degree of integration of VSLIs, a greater linewidth accuracy than previously is presently demanded for the photolithography.
As a result, the use of a short wave high-output laser such as an Excimer laser has begun to come into use in place of the super-high pressure mercury lamp.
There have been known various types of Excimer laser light sources including a so-called stable resonance type which emits a laser light which is comparatively low in temporal and spatial coherence, a so-called injection lock type which emits a laser light which is higher in temporal and spatial coherence than the stable resonance type and their modified types. However, the use of any of there types of laser light sources produces more or less spot-like illumination irregularities (interference patterns) due to the interference of the laser light.
In order to eliminate such interference patterns, U.S. Pat. No. 4,619,508 discloses a method in which the light beam is two-dimensionally scanned by a rotating mirror or the like arranged in the optical path of an illuminating system so that the spatial coherence of the light beam is substantially reduced and the interference pattern is reduced.
U.S. Pat. No. 4,851,978 granted to the inventors on Jul. 25, 1989 discloses that where a light source of the injection lock type which emits a laser beam of high temporal and spatial coherence is used, in order that any interference patterns caused in the illumination may be effectively eliminated by the vibrations of the beam, it is necessary to two-dimensionally oscillate the light beam as many times as corresponding to the number of lens elements of a fly-eye lens or the like arranged to make uniform the beam intensity distribution in the illumination system in synchronism with the laser light pulses.
In any case, however, to spatially vibrate the laser beam so as to eliminate the interference patterns as mentioned previously requires that the light beam is two-dimensionally oscillated a considerable number of times by a vibration mirror or the like during the exposure period and therefore there is the disadvantage that the exposure time must be made considerably long with the resulting considerable decrease in the production efficiency of semiconductor devices.
Moreover, the conventional methods cannot ensure the complete elimination of any interference patterns even if the light beam is oscillated a considerable number of times and they impede the formation of finer patterns of integrated ciucuits ever tending toward the realization of higher integration.